1. Technical Field
The present invention relates to a photolithography system, and more particularly, to a simulation method and system for designing an aperture in an exposure apparatus and a recording medium on which the simulation method is recorded.
1. Description
Photolithography is a process of forming a circuit pattern on a semiconductor wafer and includes a series of processes such as photoresist coating, exposure, and development. As the integration of semiconductor devices increases, various technologies related to photolithography continue to develop in order to obtain a high resolution and optimum depth of focus (DOF) of a photolithography pattern.
FIG. 1 illustrates a conventional projection exposure apparatus in which photolithography is performed. Referring to FIG. 1, for example, a pair of oval-shaped mirrors 12 are placed around a light source 11, and a collector lens 13, a fly-eye lens 14, a condenser lens 15, a photomask 16, a projection lens 17, a wafer 18, and a wafer table 19 are sequentially arranged under the light source 11. In this case, an aperture 20 which determines the direction of light, is placed between the fly-eye lens 14 and the condenser lens 15. An aperture 20 having a circular opening in the center is usually used. However, if an off-axis illumination (OAI) method is introduced, an aperture having an annular, quadropole, or quasar shaped opening is used.
The operation of the projection exposure apparatus is described below. Light emitted from the light source 11 is reflected by the oval-shaped mirror 12 and is then condensed by the collector lens 13. Light travels parallel to an optical axis through the collector lens 13 and then through the fly-eye lens 14. Light which passes through the fly-eye lens 14, passes through the aperture 20 and is condensed by the condenser lens 15. In this case, the aperture 20 has a predetermined opening as described above, and can adjust the direction of light, i.e., angle, to be transmitted to the photomask 16. Light which passes through the condenser lens 15 is transmitted to the photomask 16, which is used to form a predetermined circuit pattern. After that, light passes through the photomask 16 and the projection lens 17 and is focused on the wafer 18. As a result, the circuit pattern is projected onto the surface of the wafer 18, which is placed on the wafer table 19.
In order to obtain optimum resolution and DOF in a photolithography pattern, it is important for light to be projected in consideration of the layout of the circuit pattern of the photomask 16, and the traveling direction of light can be controlled by the aperture 20.
However, in the present integration circuit manufacturing process, at least 20 to 30 photomasks having different circuit patterns are necessary to manufacture just one device. On the other hand, only a few of the apertures 20 that are mounted in a projection exposure apparatus are used to manufacture one device. Hence, optimum resolution and DOF for each of the photomasks having the layouts of various circuit patterns cannot be obtained. Further, it is not easy to design an aperture in which optimum resolution and DOF are obtained for the layout of each of the photomasks.
To solve the above problems, it would be desirable to provide a simulation method in which an aperture is designed to obtain an optimum resolution and DOF in a photolithography pattern, taking into consideration of the layout of a circuit pattern of a photomask.
It would also be desirable to provide a simulation system implemented by the simulation method.
It would further be desirable to provide a recording medium in which the simulation method is recorded.
Accordingly, according to one aspect of the present invention, there is provided a simulation method for designing an aperture in an exposure apparatus including a light source, an optical lens group, a photomask, and an aperture.
In the method, first, the layout information of the photomask is received. Next, the aperture is divided into a plurality of pixels. Then, the pixels of the aperture are flipped, a photolithography simulation is executed to produce a simulated photolithography pattern, and the shape of the aperture that produces an optimum resolution for the simulated photolithography pattern is searched for.
More specifically, first, exposure conditions and an image of a photomask are inputted into a simulation program. Next, the simulation of a photolithography process is executed using the exposure conditions and the image of the photomask as data. Subsequently, the aperture is divided into a plurality of pixels and the pixels of the aperture are flipped. The simulation of the photolithography process is repeatedly executed when the pixels of the aperture are flipped, and a cost function value, which is a difference between the layout of the photomask and a simulated photoresist pattern produced by the simulation step, is calculated. After that, the steps of flipping the pixels of the aperture, executing the simulation of the photolithography process, and calculating a cost function value are repeatedly performed until the cost function is uniform.
As used herein, the term “flipping the pixels of the aperture” means that the pixels of the aperture are varied so that one or more of the closed pixels are opened and/or one or more of the open pixels are closed.
The exposure conditions include the wavelength of a light source in the exposure apparatus, the intensity of a light source, a numerical aperture (NA) value, and the shape of an aperture.
According to another aspect of the present invention, there is provided a simulation system for designing an aperture in an exposure apparatus. The system includes: an inputting unit adapted to receive input data, including the layout information of a photomask; a simulation executing unit which uses the input data to produce a simulated photolithography pattern and searches for the shape of an aperture providing an optimum resolution of the simulated photolithography pattern; and a display unit which displays the result of the simulation executing unit.
In this case, the input data may include exposure conditions such as numerical aperture values, the shapes of apertures, and/or the types and wavelength of light sources.
In addition, the simulation executing unit includes: a photolithography simulation unit which uses the input data to perform a simulated photolithography process; an aperture flipping unit which divides the aperture into a plurality of pixels and flips the pixels of the aperture in a stepwise manner; a cost calculating unit which calculates a cost function value, which is a difference between a photoresist pattern formed by the photolithography simulation unit and the layout of a photomask; and a cost comparing unit which determines whether or not the calculated cost function is uniform. In addition, the display unit may include a monitor.
In order to achieve the above objectives, according to another aspect of the present invention, there is provided a computer readable storage in which is stored a program for executing a method of designing an aperture in an exposure apparatus. The program includes an inputting module adapted to receive input data including the layout information of a photomask; a simulation module adapted to use the input data to perform a photolithography simulation process to produce a simulated photoresist pattern and to search for a shape of an aperture providing an optimum resolution for the simulated photoresist pattern; and a display module which controls a display of a result of the simulation module.